U.S. patent number 7,252,898 [Application Number 10/609,017] was granted by the patent office on 2007-08-07 for fuel cells comprising laminar flow induced dynamic conducting interfaces, electronic devices comprising such cells, and methods employing same.
This patent grant is currently assigned to The Board of Trustees of the University of Illinois. Invention is credited to Eric R. Choban, Paul J. A. Kenis, Larry J. Markoski.
United States Patent |
7,252,898 |
Markoski , et al. |
August 7, 2007 |
Fuel cells comprising laminar flow induced dynamic conducting
interfaces, electronic devices comprising such cells, and methods
employing same
Abstract
A fuel cell is described that includes (a) a first electrode;
(b) a second electrode; and (c) a channel contiguous with at least
a portion of the first and the second electrodes. When a first
liquid is contacted with the first electrode, a second liquid is
contacted with the second electrode, and the first and the second
liquids flow through the channel, a multistream laminar flow is
established between the first and the second liquids. Electronic
devices containing such electrochemical cells and methods for their
use are also described.
Inventors: |
Markoski; Larry J. (Champaign,
IL), Kenis; Paul J. A. (Champaign, IL), Choban; Eric
R. (Urbana, IL) |
Assignee: |
The Board of Trustees of the
University of Illinois (Urbana, IL)
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Family
ID: |
36117977 |
Appl.
No.: |
10/609,017 |
Filed: |
June 27, 2003 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20040072047 A1 |
Apr 15, 2004 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10053187 |
Jan 14, 2002 |
6713206 |
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Current U.S.
Class: |
429/448; 429/506;
429/513; 429/524; 429/526 |
Current CPC
Class: |
H01M
8/22 (20130101); H01M 8/1011 (20130101); H01M
8/08 (20130101); H01M 8/0258 (20130101); H01M
8/04186 (20130101); H01M 8/02 (20130101); Y02E
60/50 (20130101) |
Current International
Class: |
H01M
8/00 (20060101); H01M 8/04 (20060101); H01M
8/14 (20060101); H01M 8/24 (20060101) |
Field of
Search: |
;429/12,17,18,38 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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4-284889 |
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Oct 1992 |
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JP |
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10-211447 |
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Aug 1998 |
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JP |
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WO00/15872 |
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Mar 2000 |
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WO |
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WO 01/37357 |
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May 2001 |
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WO |
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WO 03/061037 |
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Jul 2003 |
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WO |
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Primary Examiner: Yuan; Dah-Wei
Attorney, Agent or Firm: Evan Law Group LLC
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATIONS
The present application is a continuation-in-part of application
Ser. No. 10/053,187, filed on Jan. 14, 2002 now U.S. Pat. No.
6,713,206, titled "Electrochemical cells comprising laminar flow
induced dynamic conducting interfaces, electronic devices
comprising such cells, and methods employing same", inventors Larry
J. Markoski et al., hereby incorporated by reference in its
entirety.
Claims
The invention claimed is:
1. A fuel cell comprising: a first electrode; a second electrode;
and a channel contiguous with at least a portion of the first and
the second electrodes; such that when a first liquid is contacted
with the first electrode, a second liquid is contacted with the
second electrode, and the first and the second liquids flow through
the channel, a multistream laminar flow is established between the
first and the second liquids, and a current density of at least 0.1
mA/cm.sup.2 is produced.
2. The fuel cell of claim 1, further comprising: the first liquid,
wherein the first liquid comprises a fuel, and the second liquid,
wherein the second liquid comprises an oxidant.
3. The fuel cell of claim 2 wherein the channel comprises a first
input adjacent to the first electrode, and a second input adjacent
to the second electrode.
4. The fuel cell of claim 3 wherein the first liquid is introduced
through the first input, and the second liquid is introduced
through the second input.
5. The fuel cell of claim 4 wherein the first liquid is introduced
through the first input using a first pump, and the second liquid
is introduced through the second input using a second pump.
6. The fuel cell of claim 3 wherein the channel further comprises a
first outlet adjacent to the first electrode and a second outlet
adjacent to the second electrode.
7. The fuel cell of claim 2, wherein: the first liquid comprises
one or more fuels selected from the group consisting of methanol,
ethanol, propanol, formaldehyde, formic acid, ferrous sulfate,
ferrous chloride, and sulfur; and the second liquid comprises one
or more oxidants selected from the group consisting of oxygen,
ozone, hydrogen peroxide, permanganate salts, manganese oxide,
fluorine, chlorine, bromine, and iodine.
8. The fuel cell of claim 2 wherein the first liquid comprises one
or more alcohol and the second liquid comprises oxygen.
9. The fuel cell of claim 8 wherein the first liquid comprises
methanol or ethanol.
10. The fuel cell of claim 2 wherein the channel has a
substantially straight flow channel geometry.
11. The fuel cell of claim 2 further comprising a support having a
surface with first and second recessed portions, wherein the first
and the second electrodes occupy the first and second recessed
portions, respectively, such that an upper surface of the first
electrode and an upper surface of the second electrode are planar
with the surface of the support.
12. The fuel cell of claim 2 wherein the first liquid and the
second liquid are immiscible.
13. The fuel cell of claim 2 wherein the first electrode and the
second electrode are spray-coated on a support.
14. The fuel cell of claim 2 wherein the first electrode and the
second electrode comprise platinum.
15. The fuel cell of claim 2 wherein at least one of the first
electrode and the second electrode comprises ruthenium.
16. The fuel cell of claim 2 wherein the first and the second
electrodes are electrically coupled.
17. The fuel cell of claim 2, further comprising a fuel sensor,
wherein the first liquid comprises a fuel whose concentration is
controlled by the fuel sensor.
18. The fuel cell of claim 2 wherein the second liquid is
mechanically coupled to a device selected from the group consisting
of a gas exchanger, an oxidant injector, an oxidant reservoir, and
combinations thereof.
19. The fuel cell of claim 2 wherein the first electrode comprises
an anode and the second electrode comprises a cathode.
20. The fuel cell of claim 2 wherein the fuel cell comprises a
direct methanol fuel cell.
21. A electronic device comprising the fuel cell of claim 2.
22. A portable electronic device comprising the fuel cell of claim
2.
23. A method of generating an electric current comprising operating
the fuel cell of claim 2.
24. A method of generating water comprising operating the fuel cell
of claim 2.
Description
BACKGROUND
This invention relates to the field of induced dynamic conducting
interfaces. More particularly, this invention relates to laminar
flow induced dynamic conducting interfaces for use in micro-fluidic
batteries, fuel cells, and photoelectric cells.
A key component in many electrochemical cells is a semi-permeable
membrane or salt bridge. One of the primary functions of these
components is to physically isolate solutions or solids having
different chemical potentials. For example, fuel cells generally
contain a semi-permeable membrane (e.g., a polymer electrolyte
membrane or PEM) that physically isolates the anode and cathode
regions while allowing ions (e.g., hydrogen ions) to pass through
the membrane. Unlike the ions, however, electrons generated at the
anode cannot pass through this membrane, but instead travel around
it by means of an external circuit. Typically, semi-permeable
membranes are polymeric in nature and have finite life cycles due
to their inherent chemical and thermal instabilities. Moreover,
such membranes typically exhibit relatively poor mechanical
properties at high temperatures and pressures, which seriously
limits their range of use.
Fuel cell technology shows great promise as an alternative energy
source for numerous applications. Several types of fuel cells have
been constructed, including: polymer electrolyte membrane fuel
cells, direct methanol fuel cells, alkaline fuel cells, phosphoric
acid fuel cells, molten carbonate fuel cells, and solid oxide fuel
cells. For a comparison of several fuel cell technologies, see Los
Alamos National Laboratory monograph LA-UR-99-3231 entitled Fuel
Cells: Green Power by Sharon Thomas and Marcia Zalbowitz, the
entire contents of which are incorporated herein by reference,
except that in the event of any inconsistent disclosure or
definition from the present application, the disclosure or
definition herein shall be deemed to prevail.
Although all fuel cells operate under similar principles, the
physical components, chemistries, and operating temperatures of the
cells vary greatly. For example, operating temperatures can vary
from room temperature to about 1000.degree. C. In mobile
applications (for example, vehicular and/or portable
microelectronic power sources), a fast-starting, low weight, and
low cost fuel cell capable of high power density is required. To
date, polymer electrolyte fuel cells (PEFCs) have been the system
of choice for such applications because of their low operating
temperatures (e.g., 60 120.degree. C.), and inherent ability for
fast start-ups.
FIG. 1 shows a cross-sectional schematic illustration of a polymer
electrolyte fuel cell 2. PEFC 2 includes a high surface area anode
4 that acts as a conductor, an anode catalyst 6 (typically platinum
alloy), a high surface area cathode 8 that acts as a conductor, a
cathode catalyst 10 (typically platinum), and a polymer electrolyte
membrane (PEM) 12 that serves as a solid electrolyte for the cell.
The PEM 12 physically separates anode 4 and cathode 8. Fuel in the
gas and/or liquid phase (typically hydrogen or an alcohol) is
brought over the anode catalyst 6 where it is oxidized to produce
protons and electrons in the case of hydrogen fuel, and protons,
electrons, and carbon dioxide in the case of an alcohol fuel. The
electrons flow through an external circuit 16 to the cathode 8
where air, oxygen, or an aqueous oxidant (e.g., peroxide) is being
constantly fed. Protons produced at the anode 4 selectively diffuse
through PEM 12 to cathode 8, where oxygen is reduced in the
presence of protons and electrons at cathode catalyst 10 to produce
water.
The PEM used in conventional PEFCs is typically composed of a
perfluorinated polymer with sulphonic acid pendant groups, such as
the material sold under the tradename NAFION by DuPont
(Fayetteville, N.C.) (see: Fuel Cell Handbook, Fifth Edition by J.
Hirschenhofer, D. Stauffer, R. Engleman, and M. Kleft, 2000,
Department of Energy--FETL, Morgantown, W. Va.; and L. Carrette, K.
A. Friedrich, and U. Stimming in Fuel Cells, 2001, 1(1), 5). The
PEM serves as catalyst support material, proton conductive layer,
and physical barrier to limit mixing between the fuel and oxidant
streams. Mixing of the two feeds would result in direct electron
transfer and loss of efficiency since a mixed potential and/or
thermal energy is generated as opposed to the desired electrical
energy.
Operating the cells at low temperature does not always prove
advantageous. For example, carbon monoxide (CO), which may be
present as an impurity in the fuel or as the incomplete oxidation
product of an alcohol, binds strongly to and "poisons" the platinum
catalyst at temperatures below about 150.degree. C. Therefore, CO
levels in the fuel stream must be kept low or removed, or the fuel
must be completely oxidized to carbon dioxide at the anode.
Strategies have been employed either to remove the impurities
(e.g., by an additional purification step) or to create CO-tolerant
electrodes (e.g., platinum alloys). In view of the difficulties in
safely storing and transporting hydrogen gas, the lower energy
density per volume of hydrogen gas as compared to liquid-phase
fuels, and the technological advances that have occurred in
preparing CO-tolerant anodes, liquid fuels have become the phase of
choice for mobile power sources.
Numerous liquid fuels are available. Notwithstanding, methanol has
emerged as being of particular importance for use in fuel cell
applications. FIG. 2 shows a cross-sectional schematic illustration
of a direct methanol fuel cell (DMFC) 18. The electrochemical half
reactions for a DMFC are as follows in acidic conditions:
##STR00001##
As shown in FIG. 2, the cell utilizes methanol fuel directly, and
does not require a preliminary reformation step. DMFCs are of
increasing interest for producing electrical energy in mobile power
(low energy) applications. However, at present, several fundamental
limitations have impeded the development and commercialization of
DMFCs.
One of the major problems associated with DMFCs is that the
semi-permeable membrane used to separate the fuel feed (i.e.,
methanol) from the oxidant feed (i.e., oxygen) is typically a
polymer electrolyte membrane (PEM) of the type developed for use
with gaseous hydrogen fuel feeds. These PEMs, in general, are not
fully impermeable to methanol. As a result, an undesirable
occurrence known as "methanol crossover" takes place, whereby
methanol travels from the anode to the cathode through the
membrane. In addition to being an inherent waste of fuel, methanol
crossover also causes depolarization losses (mixed potential) at
the cathode and, in general, leads to decreased cell
performance.
Therefore, in order to fully realize the promising potential of
DMFCs as commercially viable portable power sources, the problem of
methanol crossover must be addressed. Moreover, other improvements
are also needed including: increased cell efficiency, reduced
manufacturing costs, increased cell lifetime, and reduced cell
size/weight. In spite of massive research efforts, these problems
persist and continue to inhibit the commercialization and
development of DMFC technology.
A considerable amount of research has already been directed at
solving the aforementioned problem of methanol crossover. Solutions
have typically centered on attempts to increase the rate of
methanol consumption at the anode, and attempts to decrease the
rate of methanol diffusion to the cathode (see: A. Heinzel, and V.
M. Barragan in J. Power Sources, 1999, 84, 70, and references
therein). Strategies for increasing the rate of methanol
consumption at the anode have included increasing catalyst loading
(i.e., providing a larger surface area), increasing catalyst
activity (i.e., increasing efficiency), and raising operating
pressure and/or temperature. Strategies for decreasing the rate of
methanol diffusion to the cathode have included decreasing methanol
concentrations, fabricating thicker NAFION membranes, synthesizing
new proton conducting materials having low permeability to
methanol, lowering cell operating temperature, and fabricating
methanol tolerant cathodes. However, to date, there remain pressing
needs in DMFC technology for significantly lowered fabrication
costs, increased efficiency, extended cell lifetimes, and
appreciably reduced cell sizes/weights.
SUMMARY
The scope of the present invention is defined solely by the
appended claims, and is not affected to any degree by the
statements within this summary.
In a first aspect, the present invention provides a fuel cell that
includes (a) a first electrode; (b) a second electrode; and (c) a
channel contiguous with at least a portion of the first and the
second electrodes; such that when a first liquid is contacted with
the first electrode, a second liquid is contacted with the second
electrode, and the first and the second liquids flow through the
channel, a multistream laminar flow is established between the
first and the second liquids, and a current density of at least 0.1
mA/cm.sup.2 is produced.
In a second aspect, the present invention provides a device that
includes a fuel cell as described above.
In a third aspect, the present invention provides a portable
electronic device that includes a fuel cell as described above.
In a fourth aspect, the present invention provides a method of
generating an electric current that includes operating a fuel cell
as described above.
In a fifth aspect, the present invention provides a method of
generating water that includes operating a fuel cell as described
above.
In a sixth aspect, the present invention provides a method of
generating electricity that includes flowing a first liquid and a
second liquid through a channel in multistream laminar flow,
wherein the first liquid is in contact with a first electrode and
the second liquid is in contact with a second electrode, wherein
complementary half cell reactions take place at the first and the
second electrodes, respectively, and wherein a current density of
at least 0.1 mA/cm.sup.2 is produced.
In a seventh aspect, the present invention provides a fuel cell
that includes a first electrode and a second electrode, wherein
ions travel from the first electrode to the second electrode
without traversing a membrane, and wherein a current density of at
least 0.1 mA/cm.sup.2 is produced.
In an eighth aspect, the present invention provides the improvement
comprising replacing the membrane separating a first and a second
electrode of a fuel cell with a multistream laminar flow of a first
liquid containing a fuel in contact with the first electrode, and a
second liquid containing an oxidant in contact with the second
electrode, and providing each of the first liquid and the second
liquid with a common electrolyte.
In a ninth aspect, the present invention provides a fuel cell that
includes (a) a support having a surface; (b) a first electrode
connected to the surface of the support; (c) a second electrode
connected to the surface of the support and electrically coupled to
the first electrode; (d) a spacer connected to the surface of the
support, which spacer forms a partial enclosure around at least a
portion of the first and the second electrodes; and (e) a
microchannel contiguous with at least a portion of the first and
the second electrodes, the microchannel being defined by the
surface of the support and the spacer. When a first liquid is
contacted with the first electrode, and a second liquid is
contacted with the second electrode, a multistream laminar flow is
established between the first and the second liquids, and a current
density of at least 0.1 mA/cm.sup.2 is produced.
The presently preferred embodiments described herein may possess
one or more advantages relative to other devices and methods, which
can include but are but not limited to: reduced cost; increased
cell lifetime; reduced internal resistance of the cell; reduction
or elimination of methanol crossover or fouling of the cathode;
ability to recycle left-over methanol that crosses over into the
oxidant stream back into the fuel stream; ability to increase
reaction kinetics proportionally with temperature and/or pressure
without compromising the integrity of a membrane; and ability to
fabricate a highly efficient, inexpensive, and lightweight
cell.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a cross-sectional schematic illustration of a polymer
electrolyte fuel cell.
FIG. 2 shows a cross-sectional schematic illustration of a direct
methanol fuel cell.
FIG. 3 shows a schematic illustration of modes of fluid flow.
FIG. 4 shows a schematic illustration of the relationship between
input stream geometry and mode of fluid flow.
FIG. 5 shows a schematic illustration of the relationship between
microfluidic flow channel geometry and mode of fluid flow.
FIG. 6 shows a schematic illustration of a diffusion-based
micro-extractor.
FIG. 7 shows a schematic illustration of a direct methanol fuel
cell containing a laminar flow induced dynamic interface.
FIG. 8A shows a schematic illustration of side-by-side microfluidic
channel configuration and 8B shows a face-to-face microfluidic
channel configuration.
FIG. 9 shows a perspective view of a laminar flow fuel cell in
accordance with the present invention.
FIG. 10 shows an exploded perspective view of the fuel cell shown
in FIG. 9.
FIG. 11 shows a plot of current vs. voltage for a copper-zinc
laminar flow fuel cell.
FIG. 12 shows a plot of current vs. voltage for a platinum-platinum
laminar flow fuel cell.
FIG. 13A shows the top view of a laminar flow cell with
face-to-face electrodes, and 13B its cross-section.
FIG. 14 shows a plot of potential vs. current density plot for a
laminar fuel cell with a ferrous sulfate and potassium permanganate
fuel-oxidant combination.
FIG. 15 shows a power density to potential plot for a laminar fuel
cell with a ferrous sulfate and potassium permanganate fuel-oxidant
combination.
FIG. 16 shows a plot of potential vs. current density plot for a
laminar fuel cell with a formic acid and oxygen saturated aqueous
sulfuric acid fuel-oxidant combination.
FIG. 17 shows a power density to potential plot for a laminar fuel
cell with a formic acid and oxygen saturated aqueous sulfuric acid
fuel-oxidant combination.
FIG. 18 shows a plot of potential vs. current density plot for a
laminar fuel cell with a formic acid and potassium permanganate
fuel-oxidant combination.
FIG. 19 shows a power density to potential plot for a laminar fuel
cell with a formic acid and potassium permanganate fuel-oxidant
combination.
FIG. 20 shows a plot of potential vs. current density plot for a
laminar fuel cell with a methanol and oxygen saturated aqueous
sulfuric acid fuel-oxidant combination.
FIG. 21 shows a power density to potential plot for a laminar fuel
cell with a methanol and oxygen saturated aqueous sulfuric acid
fuel-oxidant combination.
DETAILED DESCRIPTION
A revolutionary paradigm in cell design, which solves many of the
problems described above, has been discovered wherein the use of a
PEM has been eliminated entirely. An electrochemical cell in
accordance with the present invention does not require a membrane,
and is therefore not constrained by the limitations inherent in
conventional membranes. Instead, a mechanism has been developed by
which ions can travel from one electrode to another without
traversing a membrane, and which allows proton conduction while
preventing mixing of the fuel and oxidant streams. This mechanism,
described more fully herein below, involves establishing laminar
flow induced dynamic conducting interfaces.
Throughout this description and in the appended claims, the phrase
"electrochemical cell" is to be understood in the very general
sense of any seat of electromotive force (as defined in
Fundamentals of Physics, Extended Third Edition by David Halliday
and Robert Resnick, John Wiley & Sons, New York, 1988, 662
ff.). The phrase "electrochemical cell" refers to both galvanic
(i.e., voltaic) cells and electrolytic cells, and subsumes the
definitions of batteries, fuel cells, photocells (photovoltaic
cells), thermopiles, electric generators, electrostatic generators,
solar cells, and the like. In addition, throughout this description
and in the appended claims, the phrase "complementary half cell
reactions" is to be understood in the very general sense of
oxidation and reduction reactions occurring in an electrochemical
cell.
Ideally, the structural components of a DMFC will have the
following characteristics. Preferably, the membrane should (1) be
resistant to harsh oxidizing/reducing environments; (2) possess
mechanical toughness; (3) be resistant to high temperatures and
pressures (e.g., 0 160.degree. C. and 1 10 atm); (4) be impermeable
to methanol under all operating conditions; (5) conduct protons
with minimal ohmic resistance and mass transport losses; and (6) be
composed of lightweight and inexpensive materials. Both the anode
and cathode, preferably, should (1) exhibit high catalytic
activity; (2) possess a large surface area; (3) require minimal
amounts of precious metals; and (4) be easily to fabricated. In
addition, the anode should preferably show tolerance to carbon
monoxide, and the cathode should preferably show tolerance to
methanol if so needed. The integrated fuel cell assembly itself
should preferably (1) have few moving parts; (2) require no
external cooling system; (3) require no fuel reformer or purifier;
(4) be composed of durable and inexpensive components; (5) be
easily fabricated; (6) be easily integrated into fuel cell stacks;
and (7) provide highly efficient energy conversion (i.e., at least
50%).
Heretofore, there has been no single fuel cell design that
successfully incorporates all of the aforementioned attributes.
However, it has now been discovered that by completely eliminating
the PEM from the DMFC, and by redesigning the system to function on
the microfluidic scale, one or more of these attributes can be
achieved. In the absence of a PEM, a mechanism to allow proton
conduction while preventing mixing of the fuel and oxidant streams
is needed. Such a mechanism, described more fully herein below, can
be established in microfluidic flow channels through a phenomenon
known as "multistream laminar flow," whereby two liquid streams
flow side-by-side in physical contact (thereby enabling proton
conduction), without mixing and in the complete absence of a
physical barrier or membrane. The two liquids can be miscible or
immiscible. Obviation of a physical membrane for stream segregation
and proton transport from a fuel cell significantly decreases
manufacturing costs and increases the efficiency and versatility of
the cell.
As shown in FIG. 3, fluid flow can be categorized into two regimes:
laminar flow and turbulent flow. In steady or laminar flow (FIG.
3), the velocity of the fluid at a given point does not change with
time (i.e., there are well-defined stream lines). In turbulent flow
the velocity of the fluid at a given point does change with time.
While both laminar and turbulent flow occur in natural systems
(e.g., in the circulatory system), turbulent flow generally
predominates on the macroscale. In contrast, laminar flow is
generally the norm on the microfluidic scale.
An indicator of the state of a flow stream for a fluid under flow
can be expressed as a dimensionless quantity known as the Reynolds
number (Re). The Reynolds number is defined as the ratio of
inertial forces to viscous forces, and can be expressed as:
Re=.rho.vL/.mu. where L is the characteristic length in meters,
.rho. is the density of the fluid in grams/cm.sup.3, v is the
linear velocity in meters/sec, and .mu. is the viscosity of the
fluid in grams/(sec)(cm).
There is a transitional critical value of Re for any given geometry
above which flow is said to be turbulent and below which flow is
said to be laminar. For typical fluidic devices, the transition
from laminar to turbulent flow has been empirically determined to
occur around Re=2,300. Formulae to calculate Re for specific
geometries are well known (see: Micromachined Transducers:
Sourcebook by G. T. A. Kovacs, McGraw-Hill, Boston, 1998). In some
microchannel geometries, flow is strictly laminar, reducing the
mixing of two miscible streams to the low levels due to the
interdiffusion of both liquids into each other. However, as shown
in FIG. 4, the geometry of the input streams can greatly affect
turbulence and mixing. A T-junction brings two miscible streams
together in a laminar flow, which is maintained without turbulent
mixing. In contrast, introducing the two streams in an arrow-type
junction would produce turbulent flow and subsequent mixing.
Geometry is not the only variable that affects the degree of
mixing. The residence time, or flow rates of solutions can have an
impact as well. The average time for a particle to diffuse a given
distance depends on the square of that distance. A diffusion time
scale (T.sub.d) can be expressed as T.sub.d=L.sup.2/D where L is
the relevant mixing length in micrometers and D is the diffusion
coefficient. The rate of diffusion for a given molecule is
typically determined by its size. A table of diffusion coefficients
for some common molecules is shown below in Table 1 (see: J. P.
Brody, and P. Yager, "Diffusion-Based Extraction in a
Microfabricated Device," Sensors and Actuators, January, 1997, A58,
no. 1, pp. 13 18). As may be seen from this Table, the proton
(H.sup.+) has the highest diffusion coefficient in water at room
temperature.
TABLE-US-00001 TABLE 1 Diffusion Coefficient Molecular Weight In
Water at Room Temp Water Soluble Molecule (AMU) (.mu.m.sup.2/sec)
H.sup.+ 1 9,000 Na.sup.+ 23 2,000 O.sub.2 32 1,000 Glycine 75 1,000
Hemoglobin 6 .times. 10.sup.4 70 Myosin 4 .times. 10.sup.5 10
Tobacco Mosaic Virus 4 .times. 10.sup.7 5
When two fluids with differing concentrations or compositions of
molecules are forced to flow parallel to one another in a single
channel, extraction of molecules can be accomplished on the basis
of diffusion coefficient differences. For example, as shown in FIG.
6, Na.sup.+ can be extracted from blood plasma by controlling
channel dimension, flow rate, and the dwell time the two streams
are in contact, thus producing a continuous micro-extractor (see:
Brody reference, vide supra).
It has been discovered that multistream laminar flow between two
miscible streams of liquid induces an ultra-thin dynamic conducting
("semi-permeable") interface (hereinafter "induced dynamic
conducting interface" or "IDCI"), which wholly replaces the PEMs or
salt bridges of conventional devices. The IDCI can maintain
concentration gradients over considerable flow distances and
residence times depending on the dissolved species and the
dimensions of the flow channel.
An electrochemical cell embodying features of the present invention
includes (a) a first electrode; (b) a second electrode; and (c) a
channel contiguous with at least a portion of the first and the
second electrodes. When a first liquid is contacted with the first
electrode, a second liquid is contacted with the second electrode,
and the first and the second liquids flow through the channel, a
multistream laminar flow is established between the first and the
second liquids, and a current density of at least 0.1 mA/cm.sup.2
is produced.
Flow cell designs embodying features of the present invention
introduce a new paradigm for electrochemical cells. A fuel cell 20
embodying features of the present invention that does not require a
PEM nor is subject to several of the limitations imposed by
conventional PEMs is shown in FIG. 7. In this design, both the fuel
input 22 (e.g. an aqueous solution containing MeOH and a proton
source) and the oxidant input 24 (e.g., a solution containing
dissolved oxygen or hydrogen peroxide and a proton source) are in
liquid form. By pumping the two solutions into the microchannel 26,
multistream laminar flow induces a dynamic proton-conducting
interface 28 that is maintained during fluid flow. If the flow
rates of the two fluids are kept constant and the electrodes are
properly deposited on the bottom and/or top surfaces of the
channel, the IDCI is established between anode 30 and cathode
32.
A proton gradient is created between the two streams and rapid
proton diffusion completes the circuit of the cell as protons are
produced at anode 30 and consumed at cathode 32. In this case, the
IDCI prevents the two solutions from mixing and allows rapid proton
conduction by diffusion to complete the circuit.
Preferably, the liquid containing the fuel and the liquid
containing the oxidant each contains a common electrolyte, which is
preferably a source of protons (e.g., a Bronsted acid). A portion
of these externally provided protons may be consumed in the
half-cell reaction occurring at the cathode. Thus, a reliance on
pure diffusion for conveying protons from the fuel stream to the
oxidant stream can be avoided and current densities of at least 0.1
mA/cm.sup.2 can be achieved.
Preferably, an electrochemical cell embodying features of the
present invention produces current densities of at least 0.1
mA/cm.sup.2, more preferably of at least 1 mA/cm.sup.2, still more
preferably of at least 2 mA/cm.sup.2. A current density of 27
mA/cm.sup.2 has been produced in accordance with presently
preferred embodiments. Although there is presently no preferred
limit to the amount of current density produced by an
electrochemical cell embodying features of the present invention,
it is preferred that the current density produced by a cell be
substantially matched to the requirements for a particular
application. For example, if an electrochemical cell embodying
features of the present invention is to be utilized in a cellular
phone requiring a current density of about 10 mA/cm.sup.2, it is
preferred that the electrochemical cell produce a current density
that is at least sufficient to match this demand.
Advantages of the design shown in FIG. 7 include but are not
limited to the following: reduced cost due to the elimination of a
PEM; increased cell lifetime due to the continual regeneration of
the IDCI, which neither wears out nor fails under flow; reduced
internal resistance of the cell due to the infinite thinness of the
IDCI; reduction or elimination of methanol crossover or fouling of
the cathode since, with proper design, diffusion occurs only
downstream of the cathode; ability to recycle back into the fuel
stream left-over methanol that crosses over into the oxidant
stream; ability to increase reaction kinetics proportionally with
temperature and/or pressure without compromising the integrity of
the IDCI; ability to fabricate a highly efficient, inexpensive, and
lightweight cell through optimization of cell dimensions, flow
rate, fuel (concentration and composition), oxidant (concentration
and composition) and electrodes (surface area, activity, and
chemical composition).
In an optimized cell design, the methanol is completely consumed
before it diffuses into the oxidant stream. This is feasible if the
concentration of methanol is controlled by a methanol sensor
coupled to a fuel injector or to a flow rate monitor.
Alternatively, a water immiscible oxidant fluid stream having a
very low affinity for methanol and a high affinity for oxygen and
carbon dioxide can be used in conjunction with the laminar
flow-type cell shown in FIG. 7. At least one such family of fluids
(viz., perfluorinated fluids such as perfluorodecalin available
from F2 Chemicals Ltd., Preston, UK) has been successfully used in
respiration-type fluids for medicinal applications. These fluids
exhibit an extremely high affinity for oxygen and extremely low
affinities for methanol and water. They are chemically inert and
thermally stable. When these fluids are doped with NAFION or an
alternative proton source, they become proton conducting. Thus,
inasmuch as methanol is soluble in the aqueous fuel stream only,
the unwanted problem of methanol crossover into the water
immiscible oxidant fluid stream is reduced or eliminated. Moreover,
since both liquids are excellent heat exchangers, an external
cooling system is not required.
Cell and electrode dimensions and electrode placement affect cell
efficiency. FIG. 8 shows two alternative cell designs. In FIG. 8A,
the anode and cathode are positioned side-by-side, analogous to the
placement shown in FIG. 7. In FIG. 8B, the anode and cathode are
positioned face-to-face. The optimization of cell dimensions can be
achieved via computer modeling (e.g., using fluid flow modeling
programs, Microsoft EXCEL software, etc.) to correlate optimum
laminar flow conditions (i.e., minimum mixing) with easily
fabricated channel dimensions and geometries. Critical values for
the Reynolds number can be calculated for an array of cell designs
with respect to channel width, depth, length, flow rate, and
interfacial surface area. In this manner, a channel design that
provides the greatest power output and highest fuel conversion can
be determined.
When appropriate electrode dimensions and placement of electrodes
have been determined as set forth above, the electrodes are then
patterned onto a support (e.g., a soda lime or Pyrex glass slide).
The electrodes may be sacrificial electrodes (i.e., consumed during
the operation of the electrochemical cell) or non-sacrificial
electrodes (i.e., not consumed by the operation of the
electrochemical cell). In preferred embodiments, the electrodes are
non-sacrificial. In any event, the type of electrode used in
accordance with the present invention is not limited. Any conductor
with bound catalysts that either oxidize or reduce methanol or
oxygen is preferred. Suitable electrodes include but are not
limited to carbon electrodes, platinum electrodes, palladium
electrodes, gold electrodes, conducting polymers, metals, ceramics,
and the like.
The electrode patterns can be produced by spray coating a glass
slide and mask combination with dispersions of metallic (preferably
platinum) particles in an organic or aqueous carrier. A preferred
dispersion of platinum particles in an organic carrier is the
inexpensive paint product sold under the trade name LIQUID BRIGHT
PLATINUM by Wale Apparatus (Hellertown, Pa.). The patterned slide
is then baked in a high temperature oven in the presence of oxygen
or air to produce a thin conductive layer of pure platinum. This
technique enables production of thin, high surface area,
mechanically robust, low resistance platinum electrodes on glass
slides. To increase the carbon monoxide tolerance of these
electrodes, they can be decorated with ruthenium using chemical
vapor deposition, sputtering, or a technique known as spontaneous
electroless deposition (see: A. Wieckowski et al. J. Catalysis,
2001, in press).
Once the electrodes have been patterned on a support, the
microchannel can be constructed readily from flat, inexpensive,
precision starting materials as shown in FIGS. 9 10 using
techniques such as those described by B. Zhao, J. S. Moore, and D.
J. Beebe in Science, 2001, 291, 1023 1026. Microchannel 34 can be
constructed from commercially available glass slides 36 and cover
slips 38. The microchannel 34 can be sealed with an
ultraviolet-based chemically resistant adhesive. A preferred
ultraviolet-based chemically resistant adhesive is that sold by
Norland Products, Inc. (Cranberry, N.J.), which is chemically
resistant to most water-miscible solvents. The cell thus produced
will have chemical resistance and can be employed as a single
channel laminar flow DMFC.
Once a single channel laminar flow DMFC has been assembled,
optimization experiments can be performed in which the efficiency
of the cell is evaluated with respect to concentration of methanol,
concentration of proton, oxidant composition, flow rate, and
temperature. Evaluation of cell performance is determined based on
cell potential, current density, peak power, and power output. The
single channel laminar flow DMFC is reusable, and multiple
experiments can be performed with the same cell.
The fuel and oxidant are introduced into the flow channel with the
aid of one or more pumps, preferably with the aid of one or more
high-pressure liquid chromatography (HPLC) fluid pumps. For
example, the flow rate of the fuel and oxidant streams can be
controlled with two HPLC pumps to enable precise variation of the
flow rate from 0.01 to 10 mL/min. This approach allows for the use
of large reservoirs of fuel and oxidant that can be heated to
constant temperatures and maintained under inert atmosphere, air,
or oxygen, as needed. The effluent streams can be monitored for the
presence of methanol to quantify chemical conversion, cell
efficiency, and methanol crossover, by sampling the effluent stream
and subjecting it to gas chromatographic analysis. In this manner,
the optimized operating conditions for a single channel laminar
flow DMFC can be determined.
It is noted that the fabrication technique described above can be
readily extended to the construction of multi-channel laminar flow
DMFC stacks for use in devices having increased power requirements.
Likewise, the methods described above for optimizing and
quantifying the efficiency of single channel laminar flow DMFCs can
be used to optimize and quantify the efficiency of arrayed
multi-channel cell designs. The electrodes in such multi-channel
cell designs can be connected in both series and parallel
configurations to investigate the parameters of maximum cell
voltage and current.
A single channel laminar flow DMFC can be constructed using
materials with sufficient structural integrity to withstand high
temperatures and/or pressures. Graphite composite materials
(similar to those used in DMFCs from Manhattan Scientific) or
ceramic materials (similar to those used in DMFCs from Los Alamos
National Laboratory) can be used in view of their light weight,
mechanical integrity, high temperature stability, corrosion
resistance, and low cost. In addition, a variety of fabrication
techniques can be used to produce the microchannel including
micro-milling, micro-molding, and utilizing an Electric Discharge
Machine (EDM) such as is used in the fabrication of injection
molds. The electrodes can be deposited as described above, and a
chemically inert gasket used to seal the cell. The gasket can be
made, for example, from a fluoropolymer such as
polytetrafluoroethylene sold under the trade name TEFLON by DuPont
(Wilmington, Del.). Alternative sealing techniques such as those
utilized by Manhattan Scientifics can also be employed.
Optimization and quantification of the efficiency of these single
channel laminar flow DMFCs can be achieved using the techniques
described above.
Although the manner of establishing and utilizing an induced
dynamic conducting interface in accordance with the present
invention has been described primarily in reference to a DMFC, it
is emphatically noted that the concepts and principles described
herein are general to all manner of electrochemical cells,
including but not limited to other types of fuel cells and to
batteries, photocells, and the like.
The manner in which a device embodying features of the present
invention is made, and the process by which such a device is used,
will be abundantly clear to one of ordinary skill in the art based
upon joint consideration of both the preceding description, and the
following representative procedures. It is to be understood that
many variations in the presently preferred embodiments illustrated
herein will be obvious to one of ordinary skill in the art, and
remain within the scope of the appended claims and their
equivalents.
EXAMPLES
A Laminar Flow Cell Using Sacrificial Electrodes
Flat copper and zinc electrodes (ca. 0.125.times.20.times.3 mm)
were imbedded into a block of polycarbonate by micro-machining
channels and adhering the electrodes into these channels to create
a flat surface. The electrodes were both of equivalent size and ran
parallel to each other with a gap of approximately 5 mm there
between. On top of this electrode assembly was assembled a flow
channel composed of microscope coverglass as shown in FIG. 11. The
cell was sealed with UV glue (Norland Products Inc., Cranberry,
N.J.) and the input adapters were secured with commercially
available epoxy (Loctite Quick Set Epoxy, Rocky Hill, Conn.). Once
the cell was assembled, aqueous solutions of 2M copper sulphate and
zinc sulphate were prepared. The zinc sulphate solution was brought
into the channel first over the zinc electrode with the aid of a
syringe pump (this filled the entire channel with liquid and care
was take to remove all air bubbles). The copper sulphate solution
was then introduced over the copper electrode. Laminar flow was
established between the electrodes and a current to voltage plot
was developed as shown in FIG. 11. The flow rates of the two
solutions were held constant and equal to each other (e.g., at 0.1
mL/min) in order for the induced dynamic conducting interface to
exist between the two electrodes. If the flow rates were different
and the opposing stream touched the opposite electrode, the cell
would short and produce no current. Thus, in accordance with the
present electrode configuration it is preferred that the flow rates
of the two solutions be similar (i.e., differ by less than about 15
percent, more preferably by less than about 10 percent, and still
more preferably by less than about 5 percent).
A Laminar Flow Cell Using Non-Sacrificial Electrodes
Two flat platinum electrodes (ca. 0.125.times.20.times.3 mm) were
imbedded into a block of polycarbonate by micro-machining channels
and adhering the electrodes into these channels, creating a flat
substrate with exposed electrode surfaces. The electrodes were both
of equivalent size and ran parallel to each other with a gap of
approximately 5 mm. On top of this electrode assembly was assembled
a flow channel composed of double stick tape and a microscope
coverglass as shown in FIG. 11. The cell was sealed and the input
adapters were secured with commercially available epoxy (Loctite
Quick Set Epoxy, Rocky Hill, Conn.). Next, solutions of iron (II)
chloride in 10% H.sub.2SO.sub.4 (0.6M) and potassium permanganate
in 10% H.sub.2SO.sub.4 (0.076M) were prepared. The iron solution
was brought into the channel first over the platinum electrodes
with the aid of a syringe pump (this filled the entire channel with
liquid and care was take to remove all air bubbles). The
permanganate solution was then introduced and laminar flow was
visibly established between the electrodes. The flow rates of the
two solutions were held constant and equal to each other in order
for the induced dynamic conducting interface to exist between the
two electrodes. Current flow (I) and cell potential (V) were
measured with the aid of a variable resistor and potentiometer. A
current to voltage plot was then developed as shown in FIG. 12,
thus confirming the functioning of the device as an electrochemical
cell. The flow rate was held at 0.05 mL/min and the reproducibility
was good. The power plot for this data can also be seen in FIG. 12.
The electrochemical half reactions for the cell are as follows:
##STR00002##
This particular chemistry was chosen to demonstrate the feasibility
of a reaction in which all products and reactants remained in
solution and utilized a common electrolyte. Since the electrodes
are not involved in the reaction, their lifetimes are very long and
the cell will continue to operate as long as oxidant and reductant
are provided. The IDCI has an infinite lifetime because it is
constantly being regenerated under flow. With this particular
reaction, the dark purple permanganate solution becomes colorless
at the cathode under high current conditions providing a visible
means of measuring current flow. Should the effluent stream be
purple, it indicates that oxidant has not been completely consumed.
The color change occurs only at the cathode surface (not at the
interface), further indicating true laminar flow with ion
conductivity. This technology can be transferred directly to
applications involving DMFCs.
A Laminar Flow Fuel Cell with Face-to-Face Electrodes
As seen in FIGS. 13A and 13B, the fuel cell system 1301 has the
anode and cathode electrodes in a face-to-face orientation similar
to FIG. 8B. Using a very similar fabrication scheme as described
below, the side-by-side orientation of the cathode and anode
electrodes as shown in FIG. 8A may also be obtained.
The fuel cell system 1301 includes multiple parts that are stacked
in layers. In FIG. 13 a schematic top view and a cross sectional
view is given of such a stacked layer assembly, wherein the fuel
stream 1302 and oxidant stream 1304 will convene at a Y-shaped
junction and continue to flow laminarly in parallel in the common
fluidic channel 1306 in which the catalyst covered electrodes 1308
cover part of the walls.
The central support layer 1300 that carries the outline of the
fluidic channel 1306 and supports the catalyst covered anode and
cathode electrodes 1308 with their leads 1310 may be fabricated
according to the following procedure. First, a negative of the
channel shape, a master, is obtained in thick photoresist (SU-8
series, Microchem, Newton, Mass.) via standard photolithographic
techniques using transparency films as the mask. This master is
replicated into an elastomeric mold, typically a silicone rubber
(poly(dimethylsiloxane) (PDMS) or SILGARD.TM. 184, Dow Corning,
Midland, Mich.), to obtain a positive relief structure of the
fluidic channel 1306 (for a detailed description of this type of
procedure see Duffy et al., Anal. Chem. (1998) 70, pp. 4974
4984).
The mold is replicated to obtain the desired central support layer.
For example, a liquid UV-curable polyurethane adhesive (Norland
Optical Adhesive no. 81, Norland Products, Cranbury, N.J.) is
applied and spread evenly over the elastomeric mold, then a flat
layer of the elastomeric material is applied and clamped on top (i)
to level the liquid adhesive, and (ii) to ensure that the top layer
touches the top of the positive relief fluidic channel 1306. This
clamped assembly is then treated with UV light according to the
manufacturer's instructions. Finally, the elastomeric top layer and
the positive-relief elastomeric mold are peeled away to yield a
freestanding central support structure (0.5 3 mm in thickness)
carrying the outlines (sidewalls) of the Y-shaped fluidic channel
1306 system.
Shadow evaporation of metals (for example, via an ATC 2000
sputtering system, AJA International Inc, North Scituate, Mass.) is
used to apply the anode and cathode electrodes 1308 in the
appropriate shapes to the central support layer. Typically,
chromium (usually 2 50 nm) is applied as an adhesion layer,
followed by gold (usually 50 1500 nm) as the seed layer for
consecutive electrodeposition of the catalyst, for example Platinum
Black plated on gold on each electrode separately at 2.6 V with a
current density of about 10 mA/cm.sup.2 for 3 minutes.
In the fuel cell system described herein both the anode and cathode
consist of electrodeposited Platinum Black. Similar procedures may
be used to apply other metals or combinations thereof.
The central support layer 1300 carrying the electrodes 1308 is
clamped between two gasket layers 1314 (typically 1 10 mm in
thickness) that form the top and the bottom wall of the fluidic
channel 1306 embedded in the central support structure 1300. These
two gasket layers 1314 are shaped for easy access to the leads 1310
that are connected to the electrodes 1308. Typically, slabs of a
silicone elastomer (for example PDMS) are used as gaskets. Other
materials including glass, PLEXIGLAS.TM., other gasket materials
(for example, rubber) or a combination of any of such materials
could be used as well.
To guide the fuel and oxidant into the Y-shaped channel system and
to guide the waste stream out of the channel, fluidic tubing is
placed in the gaskets. Typically, holes are punched exactly at the
three ends 1320 of the Y-shaped channel design. If the gasket
material is elastomeric (for example, PDMS) the tubes may be kept
in place by a pressure-fit mechanism.
To provide rigidity and robustness to the layered system, more
rigid top and bottom capping layers 1322 may be applied, such as 2
mm-thick PLEXIGLAS.TM.. The now five layer assembly is kept
together using clamps such as standard paper binding clips.
The procedure described above was used to manufacture a laminar
flow cell with a channel 3.0 cm long, 1.0 mm high, and 1.0 mm
electrode-to-electrode distance. This cell was used for
experimenting with the fuel-oxidant combinations reported in Table
2. All experiments were run at room temperature at a 0.5 ml/min
cell flow rate. Thus, in accordance with the present electrode
configuration (face to face), the flow rates of the fuel and
oxidant need not be equal, as even at flow rates differing by 90%
the streams cannot cross over.
TABLE-US-00002 TABLE 2 Exp. Fuel Oxidant Results A FeSO.sub.4 0.72
M KMnO.sub.4 0.144 M in FIG. 14. Load Curve in 10% 10% aqueous
H.sub.2SO.sub.4 FIG. 15. Power Density aqueous H.sub.2SO.sub.4
Curve B 10% aqueous 1 N aqueous H.sub.2SO.sub.4 FIG. 16. Load Curve
HCOOH saturated with O.sub.2 FIG. 17. Power Density C 10% aqueous
KMnO.sub.4 0.144 M in FIG. 18. Load Curve HCOOH 10% aqueous
H.sub.2SO.sub.4 FIG. 19. Power Density Curve D MeOH 1 M in 1 N
aqueous H.sub.2SO.sub.4 FIG. 20. Load Curve water saturated with
O.sub.2 FIG. 21. Power Density Curve
Saturated oxygen solutions were obtained by bubbling oxygen gas
(99.99%, S. J. Smith Welding Supply) through an aqueous solution of
1 50% H.sub.2SO.sub.4 at 298 K for at least 15 minutes. Oxygen
solutions may also be prepared by bubbling oxygen or air in aqueous
emulsions of fluorinated solvents as described in "Emulsions for
Fuel Cells", filed Jun. 27, 2003, inventors Larry J. Markoski et
al., U.S. patent application Ser. No. 10/608,815, hereby
incorporated by reference in its entirety. For example, an oxygen
solution may be obtained by bubbling oxygen gas or air in an
emulsion made by emulsifying 10 mL of perfluorodecaline (PFD) in 20
mL of 0.5 M sulfuric acid with an amount of ZONYL.RTM. FS-62
equivalent to 1% of the total weight of the emulsion.
Other examples of oxidants are solutions of ozone, hydrogen
peroxide, permanganate salts, manganese oxide, fluorine, chlorine,
bromine, and iodine. Other examples of fuels are solutions of
ethanol, propanol, formaldehyde, ferrous chloride, and sulfur.
Current flows (I) and cell potentials (V) were measured with the
aid of either a variable resistor, a potentiostat or a multimeter.
The Load Curves and Power Density plots were then developed as
shown in FIGS. 14, 15, 16, and 17, thus confirming the functioning
of the device as an electrochemical cell.
The laminar flow induced interface technology described herein is
applicable to numerous cells systems including but not limited to
batteries, fuel cells, and photoelectric cells. It is contemplated
that this technology will be especially useful in portable and
mobile fuel cell systems, such as in cellular phones, laptop
computers, DVD players, televisions, palm pilots, calculators,
pagers, hand-held video games, remote controls, tape cassettes, CD
players, AM and FM radios, audio recorders, video recorders,
cameras, digital cameras, navigation systems, wristwatches, and the
like. It is also contemplated that this technology will also be
useful in automotive and aviation systems, including systems used
in aerospace vehicles and the like.
Throughout this description and in the appended claims, it is to be
understood that elements referred to in the singular (e.g., a
microchannel, a fuel cell, a spacer, a fuel input, an oxidant
input, and the like), refer to one or a plurality of such elements,
regardless of the tense employed.
Obviously, numerous modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
* * * * *
References